Wireless communications systems use different techniques to allow many users to share simultaneously a finite radio spectrum. One technique is to split the spectrum into smaller portions and to assign one or more of these to each user. This technique is called Frequency Division Multiplexing (FDM). Another technique is to allocate to each user a unique spreading code. This technique is called Code Division Multiplexing (CDM). The third technique is to split time into small intervals and assign each user one or more of these intervals. This technique is called Time Division Multiplexing (TDM). Most wireless communications systems use a hybrid of these techniques.
TDM uses a frame and time slots within the frame. Time is split into frames, each of which is a fixed interval of time; each frame is further divided into a fixed number of time slots. A user is assigned one or more time slots for the purpose of communications.
TDM systems transmit data in a buffer-and-burst method; thus, the transmission for any user is non-continuous. For example, FIG. 6 illustrates 5 time frames each divided into 4 slots. The time frames and slots are usually all of equal width, and a broadcast using TDM would send a signal to a given terminal only within one slot, for example slot 3, of a frame. The transmission from various users can be interlaced into the repeating frame structure shown in FIG. 6. To increase the system capacity, the same radio resource, for example, time slot, frequency, or code, can be used simultaneously in the same cell or different cells.
A given terminal typically receives a transmission only during its given time slot and only from its serving base. However, a terminal can also receive signals from other sources during that time slot. These unintentionally-received signals are types of interference, and are the major limiting factors in wireless-system performance. Possible sources of interference include a transmitter in the same or neighboring cell, other base stations operating in the same or adjacent frequency band, or any other system which inadvertently leaks energy into the frequency band.
This interference is not limited to interference from direct, line-of-sight broadcasts. It can also be caused by transmitted radiation reflecting from fixed sources both inside and outside a given cell. Furthermore, atmospheric conditions may add to the various interference problems, and these effects can change randomly over time.
One way to reduce this interference is to divide the radio spectrum into different frequency sets or bands, and to assign the same set to cells that are relatively far apart. This is the concept of frequency reuse and is commonly used in current cellular systems. Cells that use the same frequency set are called co-channel cells, and the interference between signals from these cells is called co-channel interference. One drawback of this fixed reuse technique is that it reduces the capacity in each cell. Further, to support high data rate applications, it is very desirable that each cell have the ability to use fully the available radio resource.
This can be accomplished by intelligently scheduling transmissions in specific time slots to reduce interference from other concurrent transmissions, thereby achieving a high transmission-success probability. Because the interference arises from concurrent transmissions both in the same cell and from other cells, if there is a central controller in the system with global information which can coordinate the transmissions in real time, optimal performance can be achieved. However, while the assumption of a central controller is valid for single-cell wireless systems, it is not valid for multi-cell environments. This is due to the excessive communication bandwidth and processing necessary for a central controller in a multi-cell environment. Therefore, interference management among bases has to be done in a de-centralized fashion, i.e., distributed management is necessary.
A consequence of this distributed management is that the transmissions of one base can interfere with the terminals of other bases. The lack of any centralized coordination makes this interference difficult to predict or prevent. Therefore, typically, inter-cell interference is the capacity bottleneck of the overall system. The situation is exacerbated in packet-switched systems where it is very inefficient to use real-time interference sensing with feedback for the interference management.
Many techniques have been proposed for the inter-cell interference management in a broadband wireless system. Capture Division Packet Access (CDPA), which is targeted for mobile applications, uses the system resource in a completely uncoordinated fashion. A base transmits whenever it has a packet to transmit, and this packet is received successfully as long as the interference from nearby bases is acceptable. Transmission failures are taken care of by higher layer protocols such as Automatic Repeat reQuest (ARQ).
Other techniques, targeted for fixed wireless systems, have also been proposed. One algorithm, called Staggered Resource Allocation algorithm (SRA), adopts a system-planning approach for interference management in a fixed wireless system. The key concept of SRA is to identify the major sources of interference for each sector, and to schedule transmissions accordingly to avoid them. One limitation of SRA is that the identification of major interference sources is based on the geometric locations of cells and sectors. In reality, it is difficult to predict major interference sources based solely on geometry. In addition, in a real system where the bases may not be located on a hexagonal grid, designing an appropriate transmission schedule may involve an excessive amount of effort.
Another scheme, a so-called Time Slot Reuse Partitioning (TSRP) scheme, has been proposed for handling the inter-cell interference. TSRP requires the system to have more than one reuse pattern in the time domain. The different reuse patterns provide a terminal with differing levels of performance assurance which can then be used both to equalize performance across terminals and to offer different levels of performance assurance at each terminal. TSRP adopts a system-planning approach to provide a certain degree of coordination among different bases. However, TSRP is targeted for systems with a few or even one sector per base. Unfortunately, embedding a reuse pattern in TSRP implies that some of the resource cannot be reused in every cell, thus leading to a loss in efficiency.